An imaging device generally comprises a pixel matrix and reading means. Each pixel comprises at least one photosensitive element generating electric charges in proportion to the quantity of photons received. These electric charges, also called photocharges, are processed by the reading means in order to supply information representing the quantity of photons received by each photosensitive element. The reading means can be produced using CMOS technology, which allows them to be incorporated into each pixel. The pixel reading means consist for example of a so-called “charge injection” analog-to-digital conversion circuit that corresponds to the term “charge feedback digital to analog convertor”. The term “charge balancing” circuit can also be used.
A charge injection analog-to-digital conversion circuit comprises at least an integration capacitor, a comparator, a counter-charge injection circuit, and a counter. The integration capacitor is linked by one of its electrodes to the photosensitive element of the pixel under consideration. During a phase of exposure of the photosensitive element under consideration, the latter converts the photons into electron-hole pairs. The electric charges, electrons or holes, are collected by an electrode of the detector, then accumulate at the terminals of an integration capacitor, which leads to a variation of the voltage across the terminals of the capacitor. An input of the comparator is linked to the integration capacitor collecting the electric charges. The comparator compares the potential at the level of this input, called detection potential, to a threshold value. Each time the detection potential exceeds the threshold value, the signal at the output of the comparator switches from a first state to a second state. Each switch leads to the incrementation of the counter and the injection of a quantity Q0 of counter-charges on the electrode of the integration capacitor in order to compensate for the charges generated by the photosensitive element. If the quantity Q0 of counter-charges is correctly calibrated, the detection potential exceeds the threshold value again, and the output signal of the comparator switches from the second state to the first state. The switch of the output signal of the comparator and the injection of the counter-charges are repeated a certain number of times depending on the total quantity of charges generated by the photosensitive element. The number of injections of counter-charges necessary to balance the detection potential thus makes it possible to give a numerical value representing the total quantity of charges generated by the photosensitive element during a given integration time period. A drawback of this charge injection analog-to-digital conversion circuit is that it can only be adapted to a relatively limited range of doses of photons received by the photosensitive element. Indeed, in the aim of allowing the precise quantification of low doses of photons, the quantity Q0, which corresponds to the least significant bit of the value encoded by the counter, must be relatively small. However, when the quantity Q0 is relatively small, many injections must be performed to be able to quantify a large dose of photons. Thus, the counter must include a large number of bits (16 bits for example) to be able to count all the injections. This is called a “deep” counter. Furthermore, the photon stream is subject to intrinsic noise according to a Poisson law. In other words, the noise of the electric current generated by the photons is proportional to the square root of the number of photons received. Now, the photon stream can vary enormously, for example in a ratio in the order of 1 to 104. As a consequence, if the quantity Q0 is calibrated in such a way that it corresponds substantially to the lowest dose that can be received by the photosensitive element, then a large quantity of charges generated by the photosensitive element is encoded with a noise equal to several tens of times the quantity Q0. In other words, the quantity of charges is digitized with a precision greatly above the noise, which means that several bits of the counter are used pointlessly.
The patent application EP 1860778 A1 proposes a charge injection analog-to-digital conversion circuit in which the quantity Qc of counter-charges in each injection is modulated as a function of the photon stream, in this case as a function of the electric current generated by this photon stream. The greater the photon stream, the higher the quantity Qc. This stream is for example determined by the frequency of the injections. Control means make it possible to modulate the quantity Qc and to control a commutator so as to increment the counter by a number of units depending on this quantity Qc. More precisely, the commutator is controlled to increment the counter by a number of units equal to the multiple of the elementary quantity of counter-charges Q0. The precision of the digitization is therefore adapted to the detected photon streams. Furthermore, the order of magnitude of this precision is easily determined by the largest quantity Qc of counter-charges injected over a given integration time period. However, the analog-to-digital conversion circuit described in this patent application has the drawback of requiring a commutator at the input of the counter, as well as relatively complex control means. Moreover, in order to allow the digitization of doses of photons over a large range, the counter must always be as deep as that of the conversion circuit described previously.